1. Field of the Invention
This invention pertains to the welding art, and more specifically to means associated with an electric resistance spot welding system for holding relatively constant the energy flux density applied to weld sites during each of a succession of similar welding cycles.
2. Description of the Prior Art
Many systems have been proposed and used over the last century for accomplishing electrical resistance spot welding of workpieces. This type of welding is often used in conjunction with the manufacture of large metal assemblies, such as automotive bodies.
Electrical resistance spot welders include electric AC power supply circuitry capable of furnishing high current, low voltage AC electrical heating energy to the workpieces. The power is applied to the workpieces by way of electrodes comprising elongated welding tips, often made of copper alloy, which, due to their expendable nature, are removably couplable to the power circuitry. Typically, and electric spot welder is capable of applying serveral thousand amperes (amps) of current to the workpieces between the welding tips, at a potential in a range of approximately 1.5 to 6 volts at a power of about 12-25 kilowatts, for a weld cycle duration of about 0.2 to 0.4 seconds.
In spot welding systems, power is supplied to the welding tips by means of power modulating tubes known as "Ignitrons". The Ignitrons are often connected in anti-parallel, to produce a full wave AC output. Manual or automatic adjustments in the power and current produced by the power supply are made by adjusting the portion of the AC cycle during which each of the Ignitrons transmits power. As the Ignitrons are adjusted to transmit power over a comparatively larger portion of the Ac cycle, the applied power naturally rises.
Before performing a series of welds, spot welders must be manually set up. In the setup procedure, an operator performs a succession of sample welds, adjusting the applied power upwardly in steps. At some point, the applied power becomes so great that metal in the weld site area is actually blown out of the site, and an imperfect weld is obtained. This phenomenon is known as "expulsion". When expulsion occurs, the operator performing the setup procedure then knows that the applied power may be excessive and reduces the power slightly, in hopes of obtaining welds with the maximum amount of power applicable without provoking expulsion.
A typical electrical resistance spot welding system is exemplified by:
Spot welding systems of the type described above have been generally successful in welding common cold rolled steels. Such steels have a relatively wide tolerance range for welding energy, within which good quality welds can be obtained. Newer types of steels, however, coming into increasing demand, have a much narrower welding energy tolerance range within which good welds of such products can be obtained. Such products include many coated steels such as steels to which zinc is applied, either by electrode position or by dipping, in order to inhibit corrosion. These corrosion resistant coated steels are the subject of increasing demand particularly in the automotive industry, where manufacturers wish to provide broader warranties against auto body corrosion.
Spurred by the consequent demand for more precise welding energy control for coated products, many have proposed automatic energy control systems for welders. In one such system, an integrating energy monitor is employed. The energy monitor is coupled to the welding power circuitry, and responds to welding current and voltage to instantaneously integrate the amount of power applied during a welding cycle. The energy monitor thus produces a signal which indicates the instantaneous value of accumulated energy which is applied in a welding cycle.
A comparator is provided for comparing integrated energy in a weld cycle with a predetermined desired value. When the comparator circuitry senses that the predetermined energy value has been reached, it actuates circuitry for terminating the welding cycle.
An automatic welding control system incorporating an energy monitor and comparison circuitry is described in U.S. Pat. No. 4,101,753, to Buff et al, issued July 18, 1978, and entitled Flash Welding Apparatus and Method, the disclosure of which is expressly incorporated herein by reference.
Further work on improving weld consistency, a continuing problem, has been done based on empirical bases and assumptions. This work has mainly comprised an increase of operator intervention in the welding process and monitoring of welding current.
In spite of more precise welding energy control, there remain difficulties in obtaining consistently good welds over a long succession of weld cycles, particularly with the coated steel products referred to above. While the specific cause of these problems has not apparently been identified, some of them have been vaguely attributed to progressive deterioration of the weld tips which normally occurs with use.
One of these undesirable changes is known as "mushrooming" of the tips. As a tip is used to weld a coated steel product, it accretes coating material, and can actually alloy with it. In the case of welding zinc coated steel, successive welding operations can result in the accumulation of so much zinc on the tip that it alloys with the copper of the tip, and forms a generally enlarged rounded brass surface at the end of the tip. Other undesirable changes which have been thought to adversely affect welding are tip end changes which can result merely from the heating action of the welding, in which portions of the tip break away.
In order to combat weld nonuniformity, manufacturers have resorted to frequent visual inspection and replacement of weld tips by operating personnel. Additionally, sample welds are broken apart at regular intervals and visually inspected in order to ascertain whether welding quality is being maintained.
Both of these techniques are labor-intensive.
The tip and weld inspection problem is exacerbated in circumstances involving the use of so-called "robot" welding systems, which can perform multiple welds at very high rates. With higher rates of welding, weld tip deterioration occurs more rapidly, and the need for frequent visual inspection and sample checks becomes even more critical.
Welding current monitoring has been generally ineffective in ameliorating weld inconsistency.
It is therefore a general object of this invention to provide a spot welding system capable of making long successions of good quality spot welds on steel products having a relatively narrow weld energy tolerance without the need excessively frequent weld tip replacement and/or inspection.